1. Field
The present invention relates to an aberration correcting device as well as an optical head and an optical disc apparatus using the aberration correcting device.
2. Description of the Related Art
Optical discs having plural kinds of recording densities such as ones that conform to the CD (compact disc) standard or the DVD (digital versatile disk) standard have spread widely. In recent years, ultrahigh-density optical discs have been put into practical use that comply with the BD (Blu-ray disc) standard or the HD DVD (high definition digital versatile disk) standard and are further increased in recording density by recording information using blue-violet laser light.
Among optical disc apparatus which record information on an optical disc and reproducing information recorded on an optical disc are ones that employ an aberration correcting device for correcting aberrations such as coma, spherical aberration, and astigmatism that are caused by inclination of an optical disc, variation in the thickness (i.e., the distance between the outside surface and the recording layer) of an optical disc, assembling errors of optical elements and an optical head, and other factors.
For example, the DVD standard prescribes a two-layer-recording-type optical disc in which the distances between the outside surface and the first recording layer and the second recording layer of the optical disc are different from each other by about 55 μm. Therefore, a factor in causing spherical aberration, which is more essential than thickness variation of the optical disc, in inherent in the two-layer-recording-type optical disc. Coma is caused by inclination of the recording surface of an optical disk from the optical axis of an objective lens and has a component due to inclination in the tangential direction (tangential to the track of the optical disk) and a component due to inclination in the radial direction (of the optical disc).
An aberration correcting device using a liquid crystal is equipped with liquid crystal layers, glass substrates that are disposed in such a manner that the liquid crystal layers are sandwiched between them, and transparent electrodes for applying electric fields to the liquid crystal layers. Each transparent electrode is formed on that surface of the associated glass substrate which is in contact with the liquid crystal layer. Each transparent electrode is divided into plural patterns according to an aberration distribution(s). Where the number of liquid crystal layers is n (n: natural number), the number of surfaces of the transparent electrodes is 2n and the number of glass substrates is n+1. In the minimum configuration, the number of liquid crystal layers is 1, the number of surfaces of the transparent electrodes is 2, and the number of glass substrates is 2.
As the number of liquid crystal layers and the number of surfaces of the transparent electrodes increase, the number of glass substrates increases, resulting in increase in the thickness and the weight of the liquid crystal device. A thick liquid crystal device narrows the arrangement space of high-density-integrated components in an optical head, which is a factor in lowering the degree of freedom of the optical head designing. For this reason, it is desired to correct aberrations efficiently by a liquid crystal device having the above-described minimum configuration.
For example, a method has been invented in which one transparent electrode surface corrects coma in the radial direction of an optical disc and the other transparent electrode surface corrects coma in the tangential direction of the optical disc and astigmatism. The transparent electrode of the latter transparent electrode surface which corrects the coma in the tangential direction and the astigmatism may be patterned in such a manner that a central portion of the transparent electrode surface corrects the coma in the tangential direction and an outer peripheral portion corrects the astigmatism (refer to JP-A-2005-122828, for example).
For example, in the divisional patterns of the transparent electrode of JP-A-2005-122828, the pattern of the outer peripheral portion for correcting the astigmatism is not compatible with a distribution pattern of a peripheral portion of the coma in an effective range of a light beam and hence does not exhibit sufficient aberration reducing performance for the correction of the peripheral portion of the coma. Furthermore, the coma has a component in the radial direction of an optical disc and a component in the tangential direction which is perpendicular to the radial direction. This raises a disadvantage that, for example, the coma in the tangential direction cannot be corrected with high accuracy when the divisional patterns are such as to correct the coma in the radial direction.
On the other hand, one method for correcting coma and astigmatism with high accuracy with a single transparent electrode surface is to increase the number of divisional electrodes of the transparent electrode according to aberration distributions. However, although the aberration correction performance is improved by increasing the number of divisional electrodes, the increase in the number of divisional electrodes raises disadvantage that the number of signal lines is increased and a drive circuit is made complex. Therefore, divisional patterns of a transparent electrode are desired which can correct aberrations efficiently with as small a number of divisional electrodes as possible.